Modem computers are set up with various memory devices characterized by different writing speed, retention time, time of reading and access to information. This significantly complicates operation of computing systems, increasing period of preparation to work and makes more difficult information storage, etc.
One of the priorities of microelectronics is development of general-purpose memory with high speed of data writing and reading as well as long data storage period and high density of information.
At the same time potential of physical principles serving as a basis for the operation of semi-conductor microelectronic devices is largely exhausted. At present new principles of memory elements functioning are intensely searched for. The most interesting are so called hereditary technologies, these are regularly updated basic technologies, not requiring significant modification in production process.
Ferroelectric memory element (Ferroelectric RAM-FeRAM) is the most rational for the development of non-volatile memory technologies. As a mechanism of data storing in FeRAM ferroelectric effect is used. This effect is characterized by the capacity of ferroelectric material to keep electric polarization in the absence of electric field theoretically endlessly. This is determined by dipole interaction between elementary dipole moments resulting in ferroelectric normalization of dipoles, spontaneous polarization and adequate accumulation of charges on the surface of ferroelectric material. There is some parity with ferromagnetic effect that is, as it is known, widely used for memory elements development.
FeRAM memory element is constructed by setting thin ferroelectric film between two plane metallic electrodes. Its design resembles the capacitor used for the DRAM cell construction, but unlike DRAM storing data as a charge in a capacitor, ferroelectric cell stores data inside the crystalline structure. Ferroelectric crystals keep two steady polarization states—“1” and “0”. Since FeRAM cell is free from electron charge leakage causing information loss there is no necessity in recurrent data regeneration as in DRAM. Moreover in case of power-supply cut the data remain safe.
FeRAM memory elements were originally characterized by simplicity, fastness and operational safety attributable to DRAM, as well as energy independence (non-volatility) and period of data storage specific for flash-memory. Among apparent merits of FeRAM there are also radiation and resistance to other penetrating rays that is considered an Achilles heel of flash-memory.
FeRAM memory elements being technological progeny of modem memory types were portioned with their best characteristics—energy independence and fastness of operation. Ferroelectric elements of memory can be considered real contender to become the basic technology for the development of the new generation of nonvolatile memory devices. Although most problems specific for this type of memory have been already overcome, for instance the problem of material deterioration, some issues related to appliance of this technology remain unsettled.
There are a huge number of various ferroelectric cells of memory combining main elements of the memory cell: floating gate made from ferroelectric material and capacitor based on the ferroelectric material. These combinations give 4 basic types all other types of FeRAM cells present only their combinations. These are: 1T FeRAM one-transistor cell composed of the transistor with the gate made from ferroelectric material, 1C FeRAM one-capacitor cell. The most common type is 1T-1C FeRAM transistor-capacitor cell. This type of FeRAM is the closest in its structure to the memory on ferromagnetic cores. The most important positive characteristic of 1C FeRAM one-capacitor cell is a very small size of the element and correspondingly higher information capacity for the chip surface unit.
Main characteristics of the memory elements are information density expressed in the area value of the unit element measured in the units of minimum size acceptable in lithographic process (F); number of writing-reading cycles; retention time.
All types of memory cells can be divided into two main types: reading data from the memory element is followed by data destruction and needs subsequent regeneration of information analogically with the process in dynamic memory elements DRAM. This type of memory elements includes: 1C FeRAM one-capacitor cell, 1T-1C FeRAM transistor-capacitor cell and double cell 2T-2C FeRAM. Main advantage of these elements is practically unlimited time of information storage. Among limitations there are: limited number of data recording-deletion cycles, long time of access to information caused by reading method and size of memory element except for the size of 1C FeRAM one-capacitor cell with 4F2 area. This type of FeRAM is the closest in its structure to the memory on ferromagnetic cores. Area of memory element based on 1T-1C FeRAM transistor-capacitor cell is approximately 20-30 F2, this is the parameter in which it significantly concedes 1C one-capacitor and 1T one-transistor cells of ferroelectric memory.
There are elements of ferroelectric memory in which nondestructive data-reading method is applied. One of the mentioned elements is 1T FeRAM one-transistor cell characterized by its small size (5F2). However this type of memory lacks in longevity of data storage. This cell structure was used for one of the first operating FeRAM models made as far back as 1957 (U.S. Pat. No. 2,791,758, U.S. C1.340-173, 1957), (U.S. Pat. No. 3,832,700, IPC G11 c 11/22, G11c 11/40, 1974), but its characteristics did not meet the requirements to energy independent memory: charge compensation of such cell was too fast and the cell became uncontrollable.
Another important characteristic necessary for successful operation of the memory element is polarization value. For normal operation of 1T-1C FeRAM transistor-capacitor cell polarization value should be over 20 μC/cm2, sufficient polarization value for 1T FeRAM one-transistor cell is less than 0.2 μC/cm2, i.e. for normal functioning of this type of memory polarization value can be hundred times smaller (“Physics of thin film ferroelectric oxides”, Rev. Mod. Phys. V.77, p. 1083, 2005r.). Besides it is worth-while mentioning that for normal operation of 1C FeRAM transistor-capacitor cell, as well as for one-capacitor cell, the electrodes of ferroelectric memory cell should have absolute finite charge value that decreases with the decrease of minimum acceptable lithographic size (F). This requires new search of ferroelectric materials with evermore rising value of electric polarization. However it is known that increase of polarization value brings more problems with the increase of recording-reading cycles number. At the same time for 1T FeRAM one-transistor cell operation it is important to have specific surface density of electric polarization value that is rather small (less than 0.2 μC/cm2). This is undoubtedly crucial for the diminution of design standards of the lithographic process and for selection of ferroelectric materials compatible with semiconductor technology.
To sum up, there are two types of ferroelectric memory with potentially high information density of great potential interest, but they require further improvement. These are 1C FeRAM one-capacitor memory element with the area of 4F2 and 1T FeRAM one-transistor memory element with the area of approximately 5F2. Each of them has its advantages and disadvantages.
Thus, (1T FeRAM) one-transistor memory element is the most prospective among ferroelectric memory elements; it has small size and fast access to cells. Besides, this type of memory is characterized with nondestructive data-reading method, small value of electronic polarization, which results in practically unlimited number of recording-reading cycles. All this serves a prerequisite to construction of ideal memory chip based on the present memory element. However there are several significant and practically insurmountable problems for construction of ideal memory chip on the basis of (1T FeRAM) one-transistor ferroelectric memory element.
Main Problems Are As Follows
1. Duration of Data Storage
As a rule, storage period of this type of memory is several hours or days. There are publications informing on the possibility to store data for several weeks. The reach out for a period of storage even as long as a month requires complicated and faintly controllable and renewable technology of production of ferroelectric layer with very low conductivity. Anyway, this type of memory does not meet the main requirement of energy independent memory that is the storage period should be about ten years. Such short period of data storage is caused by the loss of the charge on the surface of ferroelectric layer resulting from depolarization effect and charge leakage. The latter is determined by notable conductivity of ferroelectric layer. Production of ferroelectric material with the conductivity at the level of dielectric layers of flash memory is practically impossible.
2. High Voltage During Programming
This is caused by great difference in dielectric penetrability of ferroelectric layer and buffer dielectric layers. This problem is difficult to overcome in practice.
Research on improvement of memory element based on 1T FeRAM one-transistor cell has started long ago but the problems still remain unsolved and require principally new nondestructive and fast data reading method.
At the same time 1C FeRAM one-capacitor memory element is of great interest; it can serve a basis for creating considerable memory array with the application of rather simple technology. Such memory devices can have specific but wide scope of application. However as it was specified above this type of ferroelectric memory elements has a number of weaknesses.
1. Reading method goes along with destruction of prerecorded information that extends period of accessing the memory element and reduces the number of recording-reading cycles. Development of nondestructive data-reading method is necessary.
2. High absolute value of electronic polarization is required.